UNIT 8—LESSON 2
USE OF CONSTANT-PRESSURE CHARTS
Identify the uses for the 1000-, 850-, 700-, 500-, 300-, 200-, 150-, 100-, 50-, and 25-mb constant-pressurecharts.
The 1000-mb chart
The 850-mb chart
The 700-mb chart
The 500-mb chart
The 300-mb chart
The 200-mb chart
The 150-, 100-, 50-, and 25-mb charts
Upper-air charts are termed CONSTANT-PRESSURECHARTS, because they depict conditions at levels (heights) within the atmosphere where the pressure is the same (constant).
Constant-pressure charts are produced forstandard levels. These levels are the 1,000-, 850-, 700-, 500-, 400-, 300-, 250-, 200-, 150-, 100-,70-, 50-, 30-, 20-, 10-, 7-, 5-, 3-, 2-, and 1-mb levels. However, the most commonly produced charts are for the 1000-, 850-, 700-, 500-, 300-, 200-, and 100-mb levels.
Atmospheric soundings show that pressurechanges most rapidly when the temperatures are cold and least rapidly when they are warm. Remember, pressure is a function of the weight of the atmosphere, and the atmosphere’s weight is dependent on its density. Cold air is more dense than warm air; therefore cold air is heavier and exerts more pressure at a given altitude. Assuming that two columns of air (one cold, one warm) exert the same pressure at the surface, the column containing the warm air has to extend to a greater height. Figure 8-2-1 illustrates the pressure-height differences in cold and warm air. Also, note the vertical spacing (thickness) between the constant-pressure levels.
Figure 8-2-1.—Pressure-height differences in cold and warm
Figure 8-2-1.—Pressure-height differences in cold and warmair.
As the temperatures in the atmosphere change, so do the heights of the constant-pressure levels. Based on the U.S. standard atmosphere, the approximate heights of the more common constant-pressure levels are as follows:
Constant-pressure charts are primarily used as
Constant-pressure charts are primarily used asan aid in weather forecasting. When they are used in conjunction with surface synoptic charts, the following determinations may be made:
1. Movements of weather systems
2. Areas of cyclonic and anticyclonic windflow
3. Types of air masses
4. Location of moist and dry areas within the atmosphere
5. Formation, intensification, and dissipation of pressure systems
6. Actual slopes of fronts
7. Vertical extent of pressure systems
8. Location and strength of jet streams
Learning Objective: Identify the uses ofconstant-pressure charts commonly pre-pared and displayed in most weather offices.
The following is a brief summary of theprincipal uses of constant-pressure charts commonly prepared and displayed in most weather offices.